In recent years, the use of optical fibers has become increasingly widespread in a variety of applications. For example, high capacity optical fibers are rapidly replacing traditional telephone and coaxial cable lines to transmit voice and video signals across the country. Fiber optic sensors and probes are also commonly used by physicians and researchers for such things as the treatment of cancer and for spectral analysis of samples.
The term "optical fiber" is used herein to refer generally to any optical waveguide or structure having the ability to transmit the flow of radiant energy along a path parallel to its axis and to contain the energy within or adjacent to its surface. The optical fiber category includes both "step index" and "gradient index" fibers. The term "multimode" optical fiber refers to an optical waveguide that will allow more than one bound mode to propagate. This property is typically achieved in fibers whose core diameter is relatively large (typically at least equaling about 10 microns) compared with the wavelength of the luminous radiation carried by the fiber. In contrast, "single mode" optical fibers have a core diameter on the order of magnitude of the wavelength, generally only a few microns.
In the general sense, numerical aperture (NA) refers to the vertex angle of the largest cone of meridional rays that can enter or leave an optical system or element, multiplied by the refractive index of the medium in which the vertex of the cone is located. In the context of fiber optics, numerical aperture is used to refer to the light acceptance or emergence characteristics and is a measure of light gathering ability. Numerical aperture value is often used to characterize bare, unterminated fiber, and in this context, it specifies the characteristics of the fiber with the ends polished flat. Thus, numerical aperture for conventional optical fibers has often been defined as: EQU NA=n.sub.2 sin.theta..sub.e
where n.sub.2 is the refractive index of the medium (typically air) through which the light is initially propagated so as to be incident upon an input end of the fiber, and .theta..sub.e is one half the included acceptance angle (or, conversely, the emergence angle) at the input end of that fiber. The "acceptance angle" of a fiber refers to the angles within which the input end of the fiber will accept a cone of light and undergo total internal reflection, while the "emergence angle" corresponds to the illumination pattern of light that it emitted from the output end of the fully filled fiber. Thus, an acceptance angle of .theta..sub.e indicates that the fiber will accept a cone of light within .+-..theta..sub.e. The greater the acceptance angle, the larger the light gathering ability of the optical fiber. Similarly, the fully filled fiber will have an illumination pattern defined by these angular limitations.
Numerical aperture may also be defined as a function of the physical (optical) properties of the fiber's materials of construction: ##EQU1## where n.sub.0 is the refractive index of the fiber core, and n.sub.1 is the refractive index of the cladding (the medium cylindrically encircling the core). Thus, conventional flat-faced optical fibers have numerical apertures which are primarily a function of the refractive indices of the core, cladding and media surrounding the endface.
Depending on the particular application, it may be preferable to have an optical fiber with larger or smaller angles of acceptance and emergence. For example, in certain sensing applications, it may be desirable to use an optical fiber with a relatively large acceptance angle so that the fiber will collect light more efficiently from the sample being measured. Similarly, when an optical fiber is used for illumination or indicating purposes, it is often advantageous for the light to emerge from the optical fiber with a large illumination pattern so that the light will be visible from wide viewing angles.
On the other hand, for many other applications, it may be desirable to minimize the acceptance and emergence angle of the optical fiber. For example, a fiber optic probe commonly includes at least one transmitting optical fiber that emits light into a sample to be measured and at least one adjacent receiving optical fiber that receives light reflected from the sample. By measuring the light scattered by the sample and comparing it to the source light, certain characteristics of the sample can be determined. In these cases, it is undesirable for light to pass from the transmitting fiber directly to the receiving fiber without first interacting with the sample to be measured. This criterion is difficult to meet when the fibers are positioned behind a window. To minimize this effect, it may be preferable for the transmitting fiber to have a specialized emergence pattern which projects its energy through the window and outward into the sample before rapidly diverging, and likewise for the receiving fiber.
Traditionally, the pattern and characteristics of light entering or exiting the fiber was controlled by selecting a combination of core, cladding and surrounding media such that the numerical aperture or acceptance/emergence angles were suitable for the specific application. However, there are significant physical limitations with this method that affect the ability to adjust the numerical aperture of those optical fibers. As a result, conventional flat-faced optical fibers have a relatively narrow acceptance angle, so that these fibers have poor light gathering characteristics and can often collect only a small fraction of the available light. This is particularly problematic in the case where the light beam is incident on the fiber face from extreme and highly diverse angles as the optical fiber can only accept those light rays that arrive at an angle less than or equal to its acceptance angle.
A variety of different solutions have been proposed in attempting to improve the light gathering ability of flat-faced optical fibers. Many proposals utilize discrete optical elements in front of the fiber face, such as lenses, prisms, mirrors, etc. in order to enlarge the acceptance angle of the optical fiber. However, this adds significantly to the complexity and the cost of the device, and the resulting fiber assembly is bulky and not mechanically robust. The optical elements are also inherently prone to misalignment, shifting, stresses, shock, cracks, scratches, etc.
In attempts to improve upon the displacement sensitivity in laser diode source-to-fiber coupling of conventional flat-faced optical fibers, the fiber faces of some single mode fibers have been formed into various shaped surfaces, usually spherically shaped. However, these fibers lack control of light acceptance characteristics, are constrained by the size of the fiber, are limited in their ability to collect light at wide angles, and are not effective at preventing light reflected off the endface from back propagating toward the light source.
Thus, there is a need for an improved optical fiber that provides better control of the pattern and characteristics of light entering or exiting the fiber.
There is also a need for an improved optical fiber that provides better light gathering ability, without the need for expensive and complex optical elements.